diff --git a/packages/react-scripts/lib/react-app.d.ts b/packages/react-scripts/lib/react-app.d.ts
index 8e1752555a7..649faaddf95 100644
--- a/packages/react-scripts/lib/react-app.d.ts
+++ b/packages/react-scripts/lib/react-app.d.ts
@@ -49,16 +49,16 @@ declare module '*.svg' {
 }
 
 declare module '*.module.css' {
-  const classes: { [key: string]: string };
+  const classes: { readonly [key: string]: string };
   export default classes;
 }
 
 declare module '*.module.scss' {
-  const classes: { [key: string]: string };
+  const classes: { readonly [key: string]: string };
   export default classes;
 }
 
 declare module '*.module.sass' {
-  const classes: { [key: string]: string };
+  const classes: { readonly [key: string]: string };
   export default classes;
 }